A cascaded photovoltaic grid-connected inverter includes N cascaded inverter units (such as an H-bridge inverter unit). A single phase topology of the inverter is shown in FIG. 1. A direct current side of each of the inverter units is connected to an independent photovoltaic panel, alternating current sides of the inverter units are connected in series, and then connected with a power grid through a grid-connected inductor L.
In order to improve a utilization ratio of solar energy, a Maximum Power Point Tracking (MPPT) control is independently performed on each photovoltaic panel in the cascaded photovoltaic grid-connected inverter. In a case that outputting active powers of the inverter units are seriously unbalanced due to factors such as uneven illumination, it is easy to generate a phenomenon that an inverter unit is over-modulated.
Currently, a method preventing the inverter unit from being over-modulated is to inject a reactive power to the power grid. However, injecting the reactive power to the power grid lowers a power factor of a system, and the power factor of the system may be less than a minimum permitted value.